The invention lies in the semiconductor technology field and pertains, more specifically, to a semiconductor switching element that blocks in both directions.
In order to switch currents or to apply voltages to loads, it has been known to use semiconductor switching elements, for example a MOSFET (metal oxide semiconductor field effect transistor). Areas of application are, for example, motor vehicle electrics or switched-mode power supplies, in which xe2x80x9cpower MOSFETsxe2x80x9d are used that are capable of switching the high currents or voltages which occur there.
As a result of the sequence of differently doped regions which are present in a MOSFET, namely a source region of a first conductivity type, a body region of a second conductivity type complementary to the first conductivity type, and a drain region of the first conductivity type, there is always a parasitic bipolar transistor present in the MOSFET, its base being formed by the body region and its emitter/collector being formed by the source region/drain region. In order to prevent the effects of this parasitic bipolar transistor on the dielectric strength of the MOSFET, it is conventional to short-circuit the source region and the body region of the MOSFET. This is described, for example, in the prior art cited in European patent EP 0 656 661 B1 (there, see FIG. 12).
If the source region and the body region were not short-circuited, charge carriers could accumulate in the body region during operation, that is to say when a drive potential is applied to the gate connection and a flow voltage is applied between drain and source, and would activate a parasitic bipolar transistor, which has the effect of a considerable reduction in the dielectric strength of the MOSFET. The dielectric strength of such a MOSFET in the drain-source direction is about only ⅓ of the dielectric strength of a MOSFET with a short-circuited body and source region, in which the effect of the short circuit is that source and body are always at the same potential, so that no charge carriers can accumulate in the body region.
However, short-circuiting the source and body regions has the disadvantage that the MOSFET can block only in one direction, the drain-source direction (which is usually referred to as the forward direction), while it conducts like a diode in the source-drain direction (reverse direction) when a flow voltage is applied.
In many applications, however, it is desirable to use a semiconductor switch which can block in both directions when no drive potential is present. In the case of the conventional MOSFET with a short circuit between source and body regions, this can be achieved only by means of complicated additional circuit measures.
European patent EP 0 656 661 B1 proposes replacing the short circuit by a conductive connection with a resistance, in order to increase the voltage drop across the component when a voltage is applied in the reverse direction.
It is accordingly an object of the invention to provide a two-way blocking semiconductor switching element, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which blocks in both directions and which can be implemented simply with known means.
With the foregoing and other objects in view there is provided, in accordance with the invention, a semiconductor switching element, comprising:
a control connection, a first load connection, and a second load connection;
a field effect transistor with a gate connected to the control connection, a source connected to the first load connection, a drain connected to the second load connection, and a body connection;
a bipolar transistor with a base, an emitter connected to the body connection of the field effect transistor, and a collector.
In other words, the semiconductor switching element has a field effect transistor whose gate connection is connected to a control connection of the semiconductor switching element, whose source connection is connected to a first load connection of the semiconductor switching element, and whose drain connection is connected to a second load connection of the semiconductor switching element. A bipolar transistor is connected by one emitter connection to a body connection of the field effect transistor. A base connection of the bipolar transistor and a collector connection of the bipolar transistor are preferably connected to the source connection of the field effect transistor or to the first load connection of the semiconductor switching element.
For the semiconductor switching element according to the invention, it is possible to use a field effect transistor which does not have any short circuit between body region and source, as a result of which the field effect transistor blocks in both directions and as a result of which the semiconductor switching element blocks both when a flow voltage is applied in the direction from the first to the second load connection, which corresponds to the forward direction of the field effect transistor, and when a flow voltage is applied in the direction from the second to the first load connection (the reverse direction of the field effect transistor). The bipolar transistor connected by its emitter to the body region of the field effect transistor xe2x80x9csucks awayxe2x80x9d charge carriers which accumulate in the body region of the field effect transistor. As a result, the parasitic bipolar transistor formed from the sequence of differently doped regions in the field effect transistor cannot be driven. The breakdown voltage of the field effect transistor in the forward direction, as a result of being connected to the bipolar transistor, substantially corresponds to the breakdown voltage of a corresponding field effect transistor with a short circuit between source and body regions.
If an n-channel field effect transistor is used, the bipolar transistor is a pnp bipolar transistor. If a p-channel field effect transistor is used, the bipolar transistor is an npn bipolar transistor.
The bipolar transistor is advantageously dimensioned such that it has a high current gain, preferably above 100, in order to lead charge carriers away from the body region as effectively as possible.
The field effect transistor and the bipolar transistor can be integrated in a common semiconductor body or in separate semiconductor bodies.
With the above and other objects in view there is also provided, in accordance with the invention, an integrated semiconductor switching element, comprising:
a first connection zone of a first conductivity type;
a second connection zone of the first conductivity type;
a body zone of a second conductivity type disposed between the first and second connection zones;
a control electrode adjacent the body zone and an insulating layer separating the control electrode from the body zone;
a third connection zone of the second conductivity type; and
a base zone of the first conductivity type between the body zone and the third connection zone.
In this specific implementation, there is provided an integrated semiconductor switching element, in which the field effect transistor and the bipolar transistor are integrated in a semiconductor body. It is possible, in the case of the semiconductor switching element according to the invention, for the production of the bipolar transistor to be integrated in a straightforward way into the production process of the field effect transistor.
The semiconductor switching element according to the invention has a first connection zone of a first conductivity type, a second connection zone of a first conductivity type, a body zone of a second conductivity type arranged between the first and second connection zones, and a control electrode arranged adjacent to the body zone and separated from the body zone by an insulating layer. In this case, the first and second connection zones form, for example, the source/drain zones of a field effect transistor. The control electrode forms, for example, the gate electrode of a field effect transistor. In addition, the integrated semiconductor switching element has a third connection zone of the first conductivity type and a base zone of the first conductivity type arranged between the body zone and the third connection zone. The third connection zone, which is preferably short-circuited with the first connection zone, forms the collector of the bipolar transistor, the base zone arranged between the body region and the third connection zone constituting the base of the bipolar transistor.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a semiconductor switching element that blocks in both directions, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.